1. Field of the Invention
The present invention relates to a printing system and method for forming images on both front and rear surfaces of a web and particularly to a printing system that includes a positioning control unit that controls accurate positioning of the images on both surfaces.
2. Description of the Related Art
Printing systems have been known for forming images and the like on both surfaces of a web, such as an elongated and continuous band-shaped sheet. One system that has been proposed and put into actual use includes two printing devices arranged in series. A first printing device at a front stage performs printing on a front surface of a web. After the web is discharged outside the first printing device, an inversion unit inverts the front and rear surfaces of the web. Then, the web is supplied to a second printing device at a post stage, which performs printing on a rear surface of the web.
Two types of webs are used in this system. Once type of web is a consecutive sheet formed with a row of sprocket holes on each lengthwise edge. The other type of web is a consecutive sheet with no sprocket holes. Systems that can use either type of web are becoming popular. However, when a web with no sprocket holes is used, it can be difficult to align the rear-surface image with the front-surface image.
This is particularly a problem when the first printing device is a type of printing device that forms images using electrophotographic techniques. That is, heat generated to thermally fix the toner image transferred onto the web in place can thermally shrink the web from its initial condition. As a result, the web can be shorter when fed to the second printing device.
Accordingly, because the page length when the front surface is printed on differs from the page length when the rear surface is printed on, the position of the rear-surface image formed in the second printing device will not match the position of the front-surface image formed in the first printing device.
In order to overcome the above problems, it is conceivable to use the first printing device to form positioning marks at predetermined positions on the web. The second printing device can measure the interval or detection timing of positioning marks. Then, web-transport speed in the second printing device can be controlled based on the measurement results so that position of the rear-surface image is aligned with the position of the front-surface image.
However, this conceivable configuration has some shortcomings. Positing control cannot be performed at the start of printing during the period from when web transport begins until the first positioning mark is detected. For this reason, positioning control can only be performed for a very short time when only one page, for example, is printed. For this reason, positioning control processes are stopped before operations to match positions of the front-surface and rear-surface images are completed. In the end, the problem of positional shift between the front-surface and the rear-surface images cannot be resolved. If this problem of the positioning control process being stopped midway continues, then the positional shift between the front-surface and rear-surface images accumulates, so that positional shift becomes increasingly large.
It is an objective of the present invention to overcome the above-described problems and provide a dual surface printing system and method capable of performing positioning control to match the positions of front-surface and rear-surface images even during the period from start of printing to when the first detection mark is detected. It is a further objective of the present invention to provide a dual surface printing system capable of accurately positioning front-surface and rear-surface images during an extremely short period of time.
In order to overcome the above and other objects, the present invention provides a printing system including a first printing unit and a second printing unit. The first printing unit prints images on a first surface of a web, and includes a mark forming unit that forms a positioning mark at a predetermined position of the web. The second printing unit prints images on a second surface of the web opposite from the first surface. At least the second printing unit further includes a mark detection means for detecting the positioning mark formed by the mark forming unit and outputting a mark detection signal accordingly, a calculation means for calculating an appropriate transport speed of the web based on an output timing of the mark detection signal, a memory means for storing a first information on the transport speed of the web calculated by the calculation means, and a control means for controlling a transport speed of the web based on the first information stored in the memory means at least for a period until the mark detection means detects the positioning mark for a first time after a printing operation was started.
There is also provided a printing system including a first printing unit and a second printing unit. The first printing unit prints images on a first surface of a web, and includes a mark forming unit that forms a positioning mark at a predetermined position of the web. The second printing unit prints images on a second surface of the web opposite from the first surface, and includes a transport means for transporting the web. At least the second printing unit further includes a mark detection means for detecting the positioning mark formed by the mark forming unit and outputting a mark detection signal accordingly and a control means for controlling a transport speed of the web based on an output timing of the mark detection signal. The control unit includes a microcomputer that designates a first value and a second value, a first signal process portion including a first counter that stops counting at the output timing of the mark detection signal, a second signal process portion including a second counter that is set to the first value designated by the microcomputer, the second counter outputting a pulse indicating a start timing of web transport, and a web transport control portion that controls the transport means to start transporting the web in response to the pulse from the second counter and that controls the web transport speed based on the second value. The microcomputer designates the second value based on the count value of the first counter at the time of when the first counter stops counting.
Further, there is provided a printing method for printing images on both first and second surfaces of a web. The method comprising the steps of a) forming a positioning mark at a predetermined position in addition to an image on a first surface of the web using a first printing unit, b) controlling a transport speed of the web in a second printing unit based on a first information that has been stored in a memory means, at least for a period until the positioning mark is detected in the step c) for a first time after a printing operation was started, the first information being on a transport speed of the web calculated by a calculation means during a previous printing operation, c) detecting the positioning mark using a detection unit of the second printing unit, and generating a mark detection signal accordingly, d) calculating an appropriate transport speed of the web based on an output timing of the mark detection signal, and e) updating the first information stored in the memory means.